Long-haul truck drivers have overweight and obesity rates almost 20% higher than the general population. Obesity has established mortality and disease consequences, including heart disease, diabetes, and sleep apnea. Driver health is also a public safety hazard because obesity and sleep apnea increase the risk of deadly crashes. Drivers experience multiple roadblocks to health, including laws permitting long work hours and an isolating job structure that restricts physical activity and dietary choices. Despite the growing health crisis, there is a lack of effective weight loss and health promotion interventions for truck drivers. To address this research gap we developed an innovative intervention that is integrated with the job structure and modern technologies of truck driving. Our approach uses mobile computing technologies to provide training and feedback during a weight loss competition, and delivers motivational interviewing on cell phones. Intervention feasibility and acceptability are established. In our published prospective uncontrolled trial (1) drivers made significant behavior changes and lost one unit of body mass index (M=7.8 lb [3.5 kg], p=.005, d=.68). These results are substantially better than those observed in prior studies of truck drivers. The goals of this proposal are to compare the efficacy of our intervention to a control condition, and to determine whether pre-intervention social support and stress moderate intervention efficacy. Our primary hypothesis is that the new weight loss intervention for truck drivers will produce greater change than a control condition. Our ability to successfully test this hypothesis is bolstered by our strong preliminary data, experience with the trucking population, and industry commitments to participate in the study. Our secondary hypothesis is that pre- intervention social support and stress in home and work environments will moderate intervention efficacy. This hypothesis is based on the positive effects of social support on stress levels, lifestyle behaviors, and weight loss, and on the negative effects of stress on body weight management. To test our hypotheses we have designed a cluster randomized trial where worksites (terminals) will be randomized to intervention and control conditions. We will collect measures from drivers at pre-intervention (month 0), post-intervention (month 6), and one-year follow-up (month 18). The project will take place over five years and accomplish three specific aims: (1) Determine intervention effectiveness for producing changes in diet, exercise, and body weight, (2) Determine whether pre-intervention social support and stress moderate intervention effectiveness, and (3) Measure the integrity of each intervention component and model how the intervention worked.